It is known to use in-line measuring devices containing a magneto-inductive measuring transducer to measure the flow velocity and/or volume flow, e.g. volume flow rate, of an electrically conductive fluid flowing in a stream direction through a measuring tube of the measuring transducer. For this, the magnetically inductive transducer uses, most often, diametrically opposed field coils of a magnetic circuit arrangement electrically connected to an exciter electronics of the in-line measuring device, to produce a magnetic field, which passes through the fluid within a given measuring volume at least sectionally perpendicularly to the direction of flow and that closes on itself essentially outside of the fluid. The measuring tube is made, in such case, usually of non-ferromagnetic material, so that the magnetic field is not unfavorably affected during measurement. Due to the movement of free charge carriers of the fluid in the magnetic field, an electric field is produced in the measuring volume, on the basis of the magneto-hydrodynamic principle. The electric field is directed perpendicularly to the magnetic field and perpendicularly to the direction of flow of the liquid. An electric voltage induced in the fluid is, therefore, measurable by means of at least two measuring electrodes arranged spaced from one another in the direction of the electric field, and by means of an evaluation electronics of the in-line measuring device connected to these electrodes. The induced voltage is, in turn, a measure for the volume flow rate. Fluid-contacting, galvanic, or fluid-non-contacting, capacitive, measuring electrodes can serve, for instance, to sense the induced voltage. For guiding and coupling the magnetic field into the measurement volume, the magnetic circuit arrangement generally includes coil cores surrounded by the field coils. The coil cores are separated from each other, especially diametrically, along a periphery of the measuring tube, and are arranged each with a free end face front surface, especially at positions where they are, in effect, mirror images of one another. In operation, the magnetic field created by the field coils connected to the exciter-electronics is so coupled via the coil cores into the measuring tube, that it passes through the fluid flowing between the two end faces at least sectionally perpendicularly to the stream direction.
Often used as an alternative to in-line measuring devices with magneto-inductive measuring transducers are in-line measuring devices that measure flow velocities and/or volume flow, e.g. flow rates, acoustically by means of ultrasonics, with a corresponding ultrasound transmitter- and receiver-equipped, measuring tube.
Due to the high mechanical stability demanded for such measuring tubes, these—both for magneto-inductive, as well as for acoustically measuring, transducers—comprise most often an outer, especially metal, support tube of predetermined strength and diameter, coated internally with an electrically non-conductive, insulating material of predetermined thickness, the so-called liner. For example, U.S. Pat. Nos. 6,595,069, 5,664,315, 5,280,727, 4,679,442, ,253,340, 3,213,685 or JP-Y 53-51 181 describe magneto-inductive measuring transducers, which include a measuring tube insertable fluid-tightly into a pipeline. The measuring tube, which has a first, inlet end and a second, outlet end, is comprised of a non-ferromagnetic support tube, serving as an outer casing of the measuring tube, and a tubular liner accommodated in a lumen of the support tube. The liner, which is made of an insulating material, serves to convey a flowing liquid insulated from the support tube.
The liner, which usually is made of a thermoplastic, thermosetting or elastomeric, synthetic material, or plastic, serves to isolate the support tube chemically from the fluid. In the case of magneto-inductive measuring transducers equipped with a support tube having a high electrical conductivity, for example through the use of metallic support tubes, the liner serves also as electrical isolation, or insulation, between the support tube and the fluid, for preventing a short circuiting of the electrical field through the support tube. By suitable design of the support tube, it is thus possible to match the strength of the measuring tube to the mechanical loads in particular cases of application, while an adapting of the measuring tube to the chemical and/or biological requirements of particular applications can be realized by means of the liner.
Because of its good working characteristics on the one hand, and its good chemical and mechanical properties on the other hand, polyurethane, in particular, has, alongside hard rubber or fluorine-containing synthetic materials such as PTFE, PFA, also become established as a material for liners of in-line measuring devices, especially those with magneto-inductive measuring transducers.
The polyurethanes used for the production of the described liners are mostly elastomeric plastics, that are made on the basis of liquid, multicomponent systems formed, directly before their processing, of reactive starting components. Following mixing, such a multicomponent system is applied onto the inner wall of the support tube pretreated, if necessary, with an adhesion promoter, the so-called primer, and left there to cure to form the liner within a predeterminable reaction time. A paintable or coating plastic, for example a synthetic resin lacquer, an alkyd resin lacquer, an acrylic paint, a dispersion paint, a silicate paint, an epoxy resin or the like, can serve as primer.
It is well known that polyurethanes are made by the polyaddition method from di- and poly-isocyanates and di-, or more-, valent alcohols. The starting components can, in such case, be, for example, prepolymers, composed of aliphatic and/or aromatic ether- and/or ester-groups, as well as glycol-, and isocyanate-, groups that can correspondingly react with the supplied, di-, or more-, valent, alcohol. If necessary, beyond that, powdery or pasty, if necessary also colorant, fillers, such as carbonates, silicates, soot, pigments, or reactive coloring materials, are mixed in.
Often used to manufacture liners of polyurethane is a so-called ribbon flow method, in which a previously prepared, liquid, multicomponent system is evenly distributed on the suitably moving, pre-treated if necessary, inner wall of the support tube by an appropriate pour-, or spray-, head. The reaction time required for the subsequent curing of the multicomponent system can be set by the dosage of the starting components, and, also, to a large extent by a suitable controlling of the processing temperature. However, short reaction times of less than a minute, which are necessary for cost-effective production of the liner, at a processing temperature of about room temperature, are obtained usually only through addition of a suitable catalyst, usually a heavy metal, and/or amine-containing, catalyst, to the multicomponent system. Here, especially tertiary amines and/or mercury are used as catalysts. Considering that the catalyst itself remains essentially unchanged in the finished polyurethane, the latter has then, as a result, inevitably also toxic, or at least physiologically not completely harmless, characteristics. Numerous investigations have also shown, that, especially the catalyst, can, to a significant degree, be dissolved, or leached, out of the liner, at least in the presence of water. The same is also generally to be considered true for possibly unreacted remainders of one or the other starting component or also reaction intermediate- or reaction by-products.
As a result, the polyurethanes used at present in in-line measuring devices are only suitable conditionally for applications with high hygienic requirements, e.g. for measurements in the field of drinking water, since the high requirements for fluid-touching components in the drinking water field with regard to chemical stability as well as physiological compatibility, cannot, without more, be fulfilled. Rather, comparatively expensive liners of PFA, PTFE or hard rubber are, therefore, used in the field of drinking water.
In the drinking water field, special attention is paid to, among other things, adherence to the maximally tolerable migration rate (Mmax,TOC) with regard to total organic carbon (TOC) content and/or the specific migration limit (SML) values defined for toxicologically critical substances. Equally strict are the requirements regarding the effect of the liner on the aesthetic condition of drinking water, especially regarding taste-, color-, turbidity-, and/or smell-neutrality of the liner in the presence of water, as well as regarding maximally tolerable chlorine consumption rates (Mmax,Cl).
Considering that it is not possible, without more, to preclude out-diffusing, and/or dissolution, or leaching, resulting from possible damage to the liner, of also deeper-lying constituents of the particularly applied plastic into the measured fluid over the total operating time of the inline measuring device, at least for applications in the drinking water field, not only the liner contacted by the fluid during operation, but, also, the primer possibly used for mediating between support tube and liner, must fulfill the same, strict requirements.